Interesting 
If you don’t manage to get out of FFBM you will always be brought back to that mode.
I’m an idiot, my phone is always connected to my PC when I tinker with it and I just checked, leaving FFBM only works if it’s connected via USB, strange.
Could you please try again with your phone connected to power or a PC 
BTW: Your first error wasn’t a error, you are supposed to adb reboot bootloader that brings you to what most people refer to as fastboot, adb reboot fastboot will bring you to fastboot in userspace, you will probably never need that.